PARTIAL RESISTANCE IN THE BARLEY (HORDEUM-VULGARE L) CULTIVAR CHIKURIN-IBARAKI-1 TO 2 PAV-LIKE ISOLATES OF BARLEY YELLOW-DWARF VIRUS - ALLELIC VARIABILITY AT THE YD2 GENE LOCUS

The Japanese barley cultivar, 'Chikurin Ibaraki l', is partially resis tant to the PAV serotype of barley yellow-dwarf virus (BYDV), but its induced mutant line, Ea52, is susceptible. The inheritance of resistan ce in cv. 'Chikurin Ibaraki l' to BYDV-PAV was investigated. The F-1 a nd F-2 plants of crosses of cvs 'Chikurin Ibaraki l', Ea52, 'Vixen', c arrying the Yd2 gene of resistance, and 'Plaisant', a susceptible Fren ch cultivar, were tested in growth chamber and field conditions. Isola te RG, against which 'Chikurin Ibaraki l' is partially resistant in gr owth chamber and field conditions, and isolate 2t, which overcomes the partial resistance of 'Chikurin Ibaraki l' in field conditions (Chalh oub et al. 1994) were used. The segregation of F-2 plants of crosses b etween 'Chikurin Ibaraki l' and the susceptible cultivars to isolate R G (one resistant to three susceptible) suggests that the resistance of 'Chikurin Ibaraki l' is controlled by a single recessive gene. All 53 7 F-2 plants of 'Chikurin Ibaraki 1' x 'Vixen' tested with isolate RG in growth chamber and field conditions were resistant, The F-2 plants of this cross were all resistant to isolate 2t in growth chamber condi tions but segregated with a ratio of one resistant to three susceptibl e in field conditions owing to the susceptibility of 'Chikurin Ibaraki 1' to this isolate. Results suggest that the resistance gene in 'Chik urin Ibaraki 1' is tightly linked or allelic with the Yd2 gene in 'Vix en'. However, it differs from this gene in 'Vixen' in that it can be o vercome by isolate 2t in field conditions.